The need for sensors for measuring contamination of air or other gases caused by solvents, e.g., aromatic solvents like benzene, toluene, xylenes, or e.g., aliphatic solvents or coolants/refrigerants like propane or hexane, or e.g., halocarbons, like tetrachloroethylene, dichlorodifluoromethane, 1,1,1,2-tetrafluoroethane, or 1,1,1,2,3,3,3-heptafluoropropane is increasing. Among other methods, liquid crystal (LC) layers have been used in sensing devices for such purposes. When the contaminated air or other gas (gas phase) gets into contact with a LC-phase (host phase) these contaminants can enter the host phase forming a guest phase within the host phase. The distribution of said contaminant between the gas phase and the host phase is influenced inter alia by the vapor pressure of the contaminant and by the interaction between guest and host phases. The contaminant in the host phase changes mass and viscosity of the host phase, as well as its density. These changes can be measured using a quartz micro balance (QMB) or by using acoustic wave sensors (AWS). Devices for measuring contaminants in gas phases using such principles are disclosed in Fresenius J. Analyt. Chem. 357, 27-31 (1997) and in earlier publications. So far optical effects arising from the interactions of host and guest phases used for measuring contaminants in gases are only based on measuring the absorptivity and its changes due to the interaction between host and guest phases using cholesteric LC phases. Devices containing such cholesteric LC's used for this kind of measurements are based on multiwavelength spectroscopy and are disclosed in Fresenius J. Analyt. Chem. 350, 577-581(1994). The authors of this study stress in much detail the necessity of improved algorithms necessary for data reduction in multiple wavelength absorbance measurements. It is noteworthy that later (1997) the same authors moved to non-optical principles for measuring host-guest phase interactions (see Fresenius J. Analyt. Chem. 357, 27-31 (1997) mentioned above).